Today, synthetic lubricants share with petroleum lubricants the very large world-wide lubricant market. The use of synthetic lubricants has grown significantly since the 1940's when ester, polyglycol and silicone lubricants were developed. Even earlier, about 1929, polymerized olefins were commercially used as lubricants.
Synthetic lubricants have been especially important in applications necessitating exposure to extreme temperatures since petroleum lubricating oils tend to oxidize rapidly above about 100.degree. to 125.degree. C. and to develop problems of wax separation and high viscosity at temperatures below about -20.degree. to -30.degree. C. For this reason synthetic lubricants are used, for example, in jet engines.
In applications where extreme conditions demand high performance requirements synthetic lubricants outcompete petroleum lubricants to the virtual exclusion of the latter. However, in other applications (which represent most of the lubricant market) such as in automobile engines, synthetic lubricants have captured only a modest, but growing, share. The advantages of a synthetic lubricant in automotive engines include (1) more miles between oil changes because of greater inherent thermal stability; (2) fewer temperature limitations because of lower volatility and pour points; (3) more economical use of fuel; and (4) better compliance with environmental restrictions placed upon automotive manufacturers. Because of these and other advantages it is widely believed that the use of synthetic lubricants will increase extensively over the years. New and improved synthetic lubricants are expected to contribute to this growth.
Known synthetic lubricants include certain hydrocarbons, organic esters, polyglycols, phosphates, silicones, fluorochemicals and other fluids (such as polyphenyl ethers and silicate esters). The focus of my invention is directed towards the first of the above-identified general areas, i.e. synthetic hydrocarbon lubricating oils. Hydrocarbons known and used as synthetic lubricants include alkylated benzenes, polyisobutylene oils and polymers of various other alpha olefins and of ethylene.
Two desirable properties of a synthetic lubricant are (1) a low pour point and (2) a high viscosity index.
The pour point of a lubricant represents the lowest temperature at which the lubricant will flow by gravity from a specified container under standard conditions. A low pour point is desired because it allows low temperature use of the lubricant.
A synthetic hydrocarbon lubricant having a pour point of less than about 10.degree. C. is generally desired. Preferably the pour point will be below about -10.degree. C. and most preferably below about -20.degree. C. The pour point of a lubricant can be lowered by as much as 30.degree. C. or more by adding a relatively small amount of a pour point depressant such as, for example, polymethacrylates or polymers formed by Friedel-Crafts condensation of wax with naphthalene or phenol.
The viscosity index of a lubricant is an empirical measure of the decrease in viscosity of a lubricant corresponding to increasing temperature. The smaller the change in viscosity with changing temperature the greater the viscosity index. Similarly, the greater the change in viscosity with changing temperature the smaller the viscosity index. A high viscosity index is desired because it allows use of the lubricant over a wide temperature range.
A synthetic hydrocarbon lubricant having a viscosity index of at least about 80 is generally desired. However, a lubricant having a lower viscosity index may be useful for cold weather applications if the lubricant has a very low pour point. Viscosity index improvers such as, for example, polymethacrylates and polyalkylstyrenes (having molecular weights ranging from about 45,000 to about 1,700,000), can be utilized to increase the viscosity index of a given lubricant by as much as 50 units or more.